During factory processing of semiconductor wafers to produce integrated circuits, it is customary to monitor iron contamination of wafer heating apparatus such as furnaces, by Surface Photo Voltage (SPV) measurements made upon bulk test wafers. Bulk test wafers are subjected to, for example, to thermal processing in selected apparatus. Then the iron contamination in the test wafer is measured by SPV. Excessive iron contamination in the test wafer is interpreted as indicative of furnace or process equipment contamination. Thus, the integrated circuit device yield will be lower. With the smaller device dimension and thinner gate oxide thickness, there is less tolerance to the level of contamination. With SPV technique monitoring, earlier detection of contaminants improves yields. The SPV technique measures diffusion lengths in the semiconductor substrate. Abnormally short diffusion lengths are indicative of undesirable contamination. Diffusion length measurements may be made by SPV or Deep Level Transient Spectroscopy (DLTS) techniques. DLTS measurement requires significant amount of time to prepare samples and is not always a convenient technique in manufacturing monitoring.
Unfortunately, wafers made by the Czochralski process (termed Czochralski wafers) have an undesirably high oxygen content. This oxygen precipitates during heating of the wafers and interferes with diffusion length measurements. Consequently, diffusion length measurements performed upon Czochralski test wafers may not provide a reliable indicator of iron contamination.
One solution has been to employ test wafers manufactured by the float zone process, instead of Czochralski wafers. Unfortunately, float zone wafers are very expensive and currently not widely available in eight inch diameters.
Those concerned with the development of integrated circuit processing have consistently sought improved methods evaluating iron contamination of processing.